#include<stdio.h>
#include<math.h>
int main()
{
    double a,b,c;
    scanf("%lf%lf%lf",&a,&b,&c);
    double p = 0.5*(a+b+c);
    double s = sqrt(p*(p-a)*(p-b)*(p-c));
    printf("%.1lf",s);
    return 0;
}